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In chemistry, a cluster is an array of bound atoms intermediate in character between a molecule and a solid. Carbon and boron atoms form fullerene and organoborane clusters. The other main group of clusters are metal clusters ^[1] .

Metal clusters

The oldest identified metal cluster is calomel which was known in India already in the 12th century. The existence of a mercury to mercury bond in this compound was established in beginning of the 20th century.

Metal cluster are prominently found with refractory metals. In general metal centers with large overlapping d-orbitals form stable clusters. Thus metals with a high oxidation state and therefore large effective charge tend to be unstable. Polynuclear metal carbonyls are generally found in late transition metals with low formal oxidation states. Polynuclear halides and oxides are found with early transition metals.

An often studied metal cluster compounds is dinuclear potassium octachlorodirhenate(III) or K₂Re₂Cl₈ whose peculiar molecular structure is explained by quadrupole bonding. Another dinuclear compound is di-tungsten tetra(hpp), the currect record holder low ionization energy. In trinuclear rhenium(III) chloride or (ReCl₃)_{3 the metal centers bond directly and through chlorine bridges. Because this compound is diamagnetic and not paramagnetic the rhenium bonds are double bonds and not single bonds. In the solid state further bridging occurs between neighbours and when this compound is dissolved in hydrochloric acid a Re3Cl12^3- complex forms. An example of a tetranuclear complex is hexadecamethoxytetratungsten W4(OCH3)12 with tungsten single bonds and molybdenum chloride (Mo6Cl8)Cl4 is a hexanuclear molybdenum compound and an example of an octahedral cluster. A special group of clusters with the general structure MxMo6X8 such as PbMo6S8 form a Chevrel phase which exhibit superconductivity at low temperatures.}

So-called Zintl compounds form a separate class of metal clusters. they form when poor metals are dissolved in liquid ammonia in the presence of alkali metals. Examples of Zintl anions are Bi₃^3-,Sn₉^4-,Pb₇^4- and Sb₇^3-. These anions do not require ligands and called naked clusters but are unstable nevertheless when not stabilized as salts by cryptates.

The structure of the Pb₁₀^2- anion is that of a capped square antiprism ^[2] . According to Wade's rules (2n+2) the number of cluster electrons is 22 and therefore a closo cluster. The compound is prepared from oxidation of K_{4Pb9 (itself made by heating elemental potassium and lead at 350°C) by Au⁺ in PPh3AuCl (by reaction of Hydrogen tetrachloroaurate and triphenylphosphine) in ethylene diamine with 2.2.2-crypt. This type of cluster was already known as is the endohedral Ni@Pb10^2- (the cage contains one nickel atom).}

In certain metal clusters metal aromaticity is observed.

See also

• Superatoms for certain aluminium clusters
• Gold clusters for gold clusters
• Polyhedral skeletal electron pair theory or Wade's electron counting rules

References

1. ^ Inorganic Chemistry Huyee, JE , 3rd ed. Harper and Row, New York
2. ^ The Zintl Ion [Pb₁₀]^2-: A Rare Example of a Homoatomic closo Cluster Annette Spiekermann, Stephan D. Hoffmann, Thomas F. Fässler Angewandte Chemie International Edition Volume 45, Issue 21 , Pages 3459 - 3462 2006 Abstract